The field of the invention relates to combustion engines. More specifically the invention relates to internal combustion engines that utilize late-closing intake valves.
Recently, much attention has been give to gasoline engines that operate as atmospheric Miller engines. The atmospheric Miller engine operates without a supercharger and utilizes delayed intake valve closure. The atmospheric Miller engine is of particular interest since the engine has been employed to operate at a higher expansion ratios. The higher expansion ratio, all else being equal, improves the thermal efficiency of the engine, and thus improves the fuel economy of the engine. Another beneficial aspect of the atmospheric Miller engine is that pumping losses generated by the throttle valve are reduced.
It has been discovered, for example, that a 2.6 liter Miller cycle engine equipped with rotary valves to control induced charge has similar fuel consumption characteristics as a conventional 3.0 liter Diesel engine. See Ueda et al., A Naturally Aspirated Miller Cycle Gasoline Engine--Its Capability of Emission Power and Fuel Economy, SAE Technical Paper Series, February 1996. This is particularly important since Miller-based engines are potential replacements for Diesel engines. Diesel engines, while generally more fuel efficient than their spark-ignited counterparts, have poor emission qualities. Moreover, recent scientific studies have indicated that the combustion products produced by Diesel engines include potential carcinogens. For these reasons, Miller-type engines have the potential to replace current Diesel engines that are commonly found in buses, trucks, vans, and the like where fuel economy is highly desired. Miller-type engines also have the potential to be used in hybrid automobiles such as those recently prototyped by several automobile manufacturers.
In general, an atmospheric Miller spark-ignition engine that sets the intake valve closing very late in the compression stroke and uses no throttle valve produces a higher efficiency, yet lower specific power output engine as compared to an equivalent standard engine. The overall efficiency increases due to lower pumping losses resulting from the absence of a throttle valve. The improved thermal efficiency results if a larger expansion ratio is used. The lower power output results because the late closure of the intake valve traps only a fraction of the intake mixture at the end of the compression stroke, expelling the balance of the fuel-air mixture back into the intake manifold.
This conventional atmospheric Miller engine design is disadvantageous because the engine is difficult to start. This problem is caused by inadequate vaporization of the fuel-air mixture due to low effective compression heating. In addition, the intake charge density remains essentially the same during the starting and running of the engine due to the absence of the self-regulating effect of the pressure drop across a throttle valve.
More recently, increasing attention has been given to atmospheric Miller engines that employ variable valve timing. Variable valve timing indicates that the closure time of the intake valve is varied and controlled during engine operation. Various mechanisms and methods have been employed to alter the valve timing. These include such things as adjusting the phase of the cam shaft, using rotary valves and employing mechanical linkages and the like to adjust valve timing. Engines using variable valve timing, however, suffer from a number of limitations. One particular problem is that the engine still lacks sufficient compression heating. In addition, variable valve timed Miller engines are quite complex and require a number of components. Not only does this make the engines more difficult to manufacture, this also adds frictional losses to the engine, decreasing engine efficiency. Also, small changes in the timing of the closing (often only 1.degree.-2.degree.) can significantly impact engine performance.
U.S. Pat. No. 4,917,058, issued to Nelson et al., discloses a method and apparatus for reducing pumping losses and improving brake specific fuel consumption for an internal combustion engine. The method employs variable valve timing by using a splittable cam mechanism. The engine does not use a throttle valve. During the compression stroke, the expelled inducted charge is prevented from communicating with the atmosphere by the provision of a check valve. By preventing the expelled fuel air mixture from communicating with the atmosphere, a supercharging effect is produced and fuel is conserved. The Nelson et al. device, however, still utilizes variable valve intake timing as a control mechanism. Thus, a rather complex arrangement is needed to control the engine.
Accordingly, there is a need for an atmospheric Miller engine and method of control that avoids the complexities and difficulties inherent in variable valve timed engines. A practical atmospheric Miller engine should allow the expansion and compression ratios to be sent independently and allow simple control of the combustion charge density over a wide operating range without obstructing the intake pathway. In addition, the engine would produce reasonably high specific output and not require the mechanical or manufacturing complexities of variable valve timed Miller engines.